The present invention relates to a producing method of a cellulose series polymer. Specifically, the present invention relates to a producing method of cellulose series polymer powder which is soluble in an organic solvent and a producing method of an organic solvent solution wherein a cellulose series polymer is dissolved in high concentration.
Cellulose series polymers, in particular, cellulose esters have high dimensional stability and heat resistance, and sticky as compared with synthetic resins for general use. Due to these characteristics, cellulose esters are formed to various products, e.g., films and fibers as well as used as the materials of plastics and lacquers. A cellulose ester film is representative of the support of photographic materials, and they are also used as the members of liquid crystal display (e.g., a protective film of a polarizing plate, a support of an optical compensation sheet, and a color filter) due to their optical isotropy. Moreover, a demand for biodegradable polymers has been increasingly raised in recent years from the viewpoint of the environmental protection and, as a result, the importance of cellulose series polymers have been appreciated anew.
Thus, it is thought that the importance of cellulose series polymers represented by cellulose esters will increase hereafter. On the other hand, chlorine-based solvents, such as methylene chloride, have been widely used as the solvent to make the solution of cellulose series polymers for forming films and the like, because they have a high solubility and can be easily removed at low boiling point. However, chlorine-based solvents are toxic substances such as having carcinogen or environmentally deleterious substances represented by ozone layer depletion, therefore, the use of chlorine-based solvents has been restricted very much. The discharge of methylene chloride into the air is severely restricted for the latter reason.
Accordingly, studies for solving these problems have been actively pursued in these years and some techniques have been disclosed. For example, methods of using cyclic diethers such as 1,3-dioxane and 1,3-dioxolan (e.g., JP-A-8-143708 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-A-8-323785), and methods of using fluoro-alcohols such as trifluoroethanol (e.g., JP-A-8-143709 and JP-A-11-60807) are disclosed. However, cyclic diethers of the former case are not necessarily perfect in safety and have a drawback that they are highly flammable. On the other hand, fluoro-alcohols of the latter case is expensive and it is not practicable to be used in a large quantity as the solvent for polymer formation.
Accordingly, the use of safe and inexpensive solvents such as acetone, ethanol, methyl acetate and ethyl acetate has been examined but it is difficult to dissolve cellulose series polymers, such as cellulose ester, in these solvents in concentrations required in the production of films, etc., by merely a mixing means. Therefore, a variety of means for increasing solubility are now under investigation. Specific examples of these methods include (1) a method of using a means of cooling dissolution (Makromol. Chem., 143, 105 (1971) (e.g., JP-A-9-95538 and JP-A-9-95544), (2) a method of applying extra-high pressure (e.g., JP-A-11-21379), (3) a method of using ultrasonic wave (e.g., JP-A-11-71463), and (4) a method according to a means of a two-component system mixed fluid under high pressure of acetone, triacetin, methanol or ethanol and carbon dioxide (e.g., U.S. Pat. No. 5,512,231 and Journal of Supercritical Fluids, 13, 135 (1998)).
The above methods (1) to (4) are certainly effective for improving solubility but there are big problems in every of them from the viewpoint of an industrial producing method. That is, for obtaining sufficient solubility according to method (1), an extremely low temperature such as xe2x88x9270xc2x0 C. is necessary, which requires a vast sum of facility investment. For obtaining sufficient solubility according to method (2), extra-high pressure of almost 100 MPa is necessary, hence the realization of practical use is also industrially difficult in view of facilities. In method (3), it is necessary to develop an ultrasonic wave generating apparatus which can be used in an industrial scale and it is considerably practicably difficult at this point of time. In method (4), a sufficient amount of cellulose series polymers cannot be dissolved, and the stability of the solution with the lapse of time is low, and partial gelation is occur.
Thus, it can be said that an industrially practicable technique of dissolving cellulose series polymers without using environmentally deleterious chlorine-based solvents to form films with small facility investment has not yet been found. Therefore, such is the state of things that the development of a novel method is strongly desired.
Accordingly, an object of the present invention is to provide a producing method of cellulose series polymers, in particular, cellulose ester powder or the organic solvent solution of cellulose ester which is high in safety, small in environmental load, and light in imposition of facility investment (i.e., inexpensive).
The present inventors have repeated examinations for solving the above problems. As a result, the present inventors have found that when a cellulose polymer is dispersed and stirred in an organic solvent other than chlorine-based polymers and liquid carbon dioxide is added to the dispersion in an appropriate weight ratio and stirred, the polymer is dissolved, and the polymer once dissolved does not precipitate and dissolved in the organic solvent after the carbon dioxide is removed, and that the powder obtained by removing the organic solvent has excellent solubility as compared with polymers not subjected to this treatment. The present invention has been attained based on the above finding.
That is, the present invention provides the following.
(1) A process for producing a cellulose polymer powder which comprises the steps of dissolving a cellulose polymer by exposure to an organic solvent containing, as the main component, an ester-based solvent having from 2 to 8 carbon atoms, a ketone-based solvent having from 4 to 10 carbon atoms, or an alcohol-based solvent having from 3 to 6 carbon atoms, and carbon dioxide, and then removing the carbon dioxide and the organic solvent from the solution.
(2) A process for producing an organic solvent solution of a cellulose polymer which comprises dissolving a cellulose polymer by exposure to an organic solvent containing, as the main component, an ester-based solvent having from 2 to 8 carbon atoms, a ketone-based solvent having from 4 to 10 carbon atoms, or an alcohol-based solvent having from 3 to 6 carbon atoms, and carbon dioxide.
(3) The process as described in the above item (1) or (2), wherein the cellulose polymer is dissolved at the temperature and the pressure not lower than the critical temperature and the critical pressure of carbon dioxide.
(4) The process as described in the above item (1) or (2), wherein the cellulose polymer is cellulose acetate having the degree of acetylation of 55.0% or more and the concentration of the polymer in the organic solvent solution is 10 mass % or more.
As the preferred embodiments of the above-described present invention, the followings can be exemplified.
(5) The process as described in the above item (1) or (2), wherein said cellulose polymer is cellulose ester.
(6) The process as described in the above item (5), wherein said cellulose ester is acetyl cellulose.
(7) The process as described in the above item (6), wherein said acetyl cellulose has the acetylation degree of 58% or more.